1. Field of the Invention
This invention relates to a variable-speed control apparatus for an induction motor, or more in particular to a variable-speed control apparatus suitable for sudden acceleration or deceleration of an induction motor.
2. Description of the Prior Art
It is well known that, in order to effect a variable-speed operation of an induction motor, the induction motor is connected to a variable-voltage variable-frequency power supply and the output voltage and the output frequency of the power supply is changed. In this case, the output voltage and the frequency are changed while maintaining constant the ratio of the voltage V to the frequency f of the motor, i.e., the ratio of the output voltage to the frequency of the power supply (hereinafter which is called the ratio V/f). This is for maintaining the torque of the induction motor constant and for preventing magnetic saturation of the core of the induction motor. In the low-frequency range of the output frequency, a control for a constant torque of the induction motor is effected by compensating for a voltage drop due to the impedance of the primary winding of the induction motor. This system may be satisfactorily applied to the case in which the induction motor is not subjected to sudden acceleration or deceleration.
In the case of involving sudden acceleration or deceleration of a large-inertia load by an induction motor such as in driving table rollers of a mill, however, the input current of the induction motor undergoes a change in magnitude ten and several times the normal value, and also the power factor is subjected to a great change from positive to negative. In such a case, keeping the ratio V/f of the induction motor constant is not sufficient for maintaining the torque constant and preventing magnetic saturation of the core of the induction motor. In order to overcome this problem, the ratio of an induced voltage Vm to the frequency f of the induction motor (hereinafter which is called the ratio Vm/f) must be kept constant. The conventional control apparatus involving only the constancy of the ratio V/f results in a shortage of torque generation or an overexcitation of the induction motor, thereby leading to a disadvantage of low responsivness to a speed command.